The semiconductor industry currently uses different types of semiconductor-based imagers, such as charge coupled devices (CCDs), photodiode arrays, charge injection devices and hybrid focal plane arrays, among others.
Because of the inherent limitations and expense of CCD technology, CMOS imagers have been increasingly used as low cost imaging devices. A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including either a photodiode, a photogate or a photoconductor overlying a doped region of a substrate for accumulating photo-generated charge in the underlying portion of the substrate. A readout circuit is connected to each pixel cell and includes a charge transfer section formed on the substrate adjacent the photodiode, photogate or photoconductor having a charge sensing node, typically a floating diffusion node, connected to the gate of a source follower output transistor. The imager may include at least one transistor for transferring charge from the charge accumulation region of the substrate to the floating diffusion node and also has a transistor for resetting the diffusion node to a predetermined charge level prior to charge transfer.
In a conventional CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the floating diffusion node; (4) resetting the floating diffusion node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. The charge at the floating diffusion node is converted to a pixel output voltage by the source follower output transistor. The photosensitive element of a CMOS imager pixel is typically either a depleted p-n junction photodiode or a field induced depletion region beneath a photogate.
CMOS imaging circuits of the type discussed above are generally known and discussed in, for example, Nixon et al., “256.times.256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994), the disclosures of which are incorporated by reference herein.
A schematic top view of a semiconductor wafer fragment of an exemplary CMOS sensor pixel four-transistor (4T) cell 10 is illustrated in FIG. 1. As it will be described below, the CMOS sensor pixel cell 10 includes a photo-generated charge accumulating area 21 in an underlying portion of the substrate. This area 21 is formed as a pinned photodiode 11, shown in FIG. 2, formed as part of a p-n-p structure within a substrate 20. The pinned photodiode is termed “pinned” because the potential in the photodiode is pinned to a constant value when the photodiode is fully depleted. It should be understood, however, that the CMOS sensor pixel cell 10 may include a photogate, photoconductor or other image to charge converting device, in lieu of a pinned photodiode, as the initial accumulating area 21 for photo-generated charge.
The CMOS image sensor 10 of FIG. 1 has a transfer gate 30 for transferring photoelectric charges generated in the charge accumulating region 21 to a floating diffusion region (sensing node) 25. The floating diffusion region 25 is further connected to a gate 50 of a source follower transistor. The source follower transistor provides an output signal to a row select access transistor having gate 60 for selectively gating the output signal to terminal 32. A reset transistor having gate 40 resets the floating diffusion region 25 to a specified charge level before each charge transfer from the charge accumulating region 21.
The charge accumulating region 21 is formed as a pinned p-n-p photodiode 11 which has a p-type layer 24, an n-type region 26 within the p-type substrate 20. The pinned photodiode 11 includes two p-type regions 20, 24 and the n-type photodiode region 26 which is fully depleted at a pinning voltage. Impurity doped source/drain regions 22 (FIG. 1), preferably having n-type conductivity, are provided on either side of the transistor gates 40, 50, 60. The floating diffusion region 25 adjacent the transfer gate 30 is also preferable n-type.
FIG. 2 also illustrates trench isolation regions 15 formed in the active layer 20 adjacent the charge accumulating region 21. The trench isolation regions 15 are typically formed using a conventional STI process or by using a Local Oxidation of Silicon (LOCOS) process. A translucent or transparent insulating layer 55 formed over the CMOS image sensor 10 is also illustrated in FIG. 2. Conventional processing methods are used to form, for example, contacts 32 (FIG. 1) in the insulating layer 55 to provide an electrical connection to the source/drain regions 22, the floating diffusion region 25, and other wiring to connect to gates and other connections in the CMOS image sensor 10.
Generally, in CMOS image sensors such as the CMOS image sensor cell 10 of FIGS. 1-2, incident light causes electrons to collect in region 26. A maximum output signal, which is produced by the source follower transistor having gate 50, is proportional to the number of electrons to be extracted from the region 26. The maximum output signal increases with increased capacitance or acceptability of the region 26 to acquire electrons. The capacitance of pinned photodiode region typically depends on the doping concentration of impurities implanted into the active layer.
PNP photodiodes, such as the pinned photodiode 11 of FIG. 2, are becoming increasingly popular for high efficiency image sensors, particularly for image sensors operating at smaller wavelengths of the visible spectrum, for example, at the blue wavelength. Nevertheless, photodiodes for the blue spectrum are complex to design and have critical requirements for the potential barriers and wells located adjacent the transfer gate of the transfer transistor. This is partially because short-wavelength blue photons are absorbed closer to the surface of a substrate in a photodiode, as compared to either the red or green photons which are absorbed deeper.
In addition, the minority carriers in a blue pixel sensor cell are substantially more likely to be lost in recombination than the minority carriers formed in the red and green pixel sensor cells. The difference in the recombination rates is due to the relatively shallow penetration depths of the blue photons, the higher majority carrier concentration that exists in the n+ region 26 than in the substrate 20, and the depth of the junction. For example, even though the average penetration of a blue photon in a CMOS photodiode is approximately 0.2μ, a large number of blue photons fail to penetrate beyond the 0.1μ junction. This way, a large amount of these photons are lost to recombinations and the blue cell response remains substantially below the red cell and green cell responses. For these reasons, it is desirable for p-n junctions, such as the p-n junction between the p-type pinned layer 24 and the n-type region 26 of FIG. 2, to be very shallow.
In addition, it is desirable for the top surface layer (p-type) of the photodiode not to get depleted when the photodiode is pinned. In other words, the top surface layer should not get depleted when the main photodiode region gets depleted. This is to ensure that the leakage arising due to surface effects (defects, poor passivation, etc) do not contribute to the photodiode response characteristics. For example, surface defects occur as a result of transient-enhanced diffusion (TED) which, in turn, is the result of interstitials and extended defects due to implanted ions during implantation. Reducing the leakage arising due to the surface defects, particularly the TED defects, necessitates a high doping concentration in the top layer while, at the same time, maintaining a shallow junction.
There is needed, therefore, a shallow and highly concentrated pinned surface layer of a photodiode for an improved high blue response photosensor with suppressed transient-enhanced diffusion. There is also needed an active pixel photosensor for use in a CMOS imager that exhibits improved color separation, a better signal-to-noise ratio, minimized dark current and reduced cross-talk. A method of fabricating an active pixel photosensor exhibiting these improvements is also needed.